This invention relates generally to radomes and like RF-transparent shield structures, and more particularly to radomes adapted to function under severe environmental conditions involving high thermal and mechanical stress levels.
The RF-transparent shield structures of the present invention have utility in a variety of applications such as in radomes for high speed aircraft and radomes for ground radars which must be "hardened" against nuclear radiation and overpressure. They offer particular advantage for use as missile nose cone radomes, in which the radome serves both as a radar window and to define the aerodynamic profile for the missile forward end. In this application the radome must provide high transmission efficiency and low insertion phase distortion over a wide range of radiation incidence angles, and must do so irrespective of polarization of the radiation. Further it must accomplish this in the presence of a severe thermal heating environment and high aerodynamic and "g" loading forces, so the radome must be very strong mechanically, be capable of withstanding thermal shock, and be able to maintain its high transmission efficiency and low phase distortion under high temperature conditions.
Among prior art radome proposals for the missile nose cone application are all-dielectric radome structures. The principal difficulty with such solid dielectric radomes is the present unavailability of ceramics or other dielectric materials which combine the necessary mechanical and electrical characteristics. For example, materials such as beryllium oxide and silicon nitride provide relatively good mechanical strength if of sufficient thickness, and have good erosion resistance. However, these materials are lacking in thermal shock resistance in the most severe environments, and the thick walls required for strength are too thick for good electrical characteristics at millimeter wavelengths. Also,presently available beryllium oxides exhibit excessive change in dielectric constant with temperature, and silicon nitrides are excessively lossy particularly at high temperatures. Materials such as fused quartz and boron nitride, which offer more compatible electrical characteristics, generally do not meet mechanical and thermal stress requirements when fabricated into bodies of the size required for all-dielectric radome applications.
Other candidate missile radome designs include that disclosed in U.S. Pat. No. 3,975,738 to Pelton et al, employing a thin sheet of metal apertured by a plurality of "tripole" configured slots disposed in a triangular grid structure. Such thin metal structures are not themselves capable of withstanding the mechanical and thermal stresses encountered in the missile nose cone application, and when interleaved or otherwise integrated into a multilayer ceramic nose cone structure they are subject to the problems noted above with respect to the all-dielectric radome design. Two other RF-shield structures are disclosed in U.S. Pat. Nos. 3,310,808 to Friis and 3,448,455 to Alfandari et al. Both these designs employ dielectric rods which project beyond the surfaces of the metal wall structures in which the rods are disposed, and as a consequence these designs are not suitable for the missile nose cone and similarly demanding applications.
The present invention has as its principal objective the provision of a radome design capable of surviving extreme environmental conditions as encountered in missile and similar applications, and capable of meeting electrical performance requirements notwithstanding those conditions. One such requirement commonly encountered is that the energy propagating through the radome be attenuated relatively little thereby, and that both this attenuation and the insertion phase introduced by the radome be as constant as possible for all radiation incidence angles as well as for two orthogonal polarizations of the incident radiation. To achieve these objectives the conditions which must be satisfied insofar as possible are that the equivalent electrical thickness of the radome remain essentially constant and be independent of polarization and incidence angle, and that the insertion phase be also independent of polarization and incidence angle. Additionally, for the environmentally severe applications under consideration, the radome structure must be adequate to withstand large mechanical forces, strong thermal shock and high temperatures. Although all these desiderata cannot be completely satisfied under all conditions, the radome of the present invention provides a useful and workable solution well adapted to many difficult application requirements.